The (low-lying) coastal zones of the world are facing increased pressure due to climate change and response to human interventions. Sea level rise, intensified storms, and the grey engineered interventions such as seawalls and dykes are contributing to degradation of natural coastal protection systems like mangrove ecosystems. Despite the well-known values of mangrove ecosystems offering natural coastal protection by trapping sediment, storing carbon, and provide many ecosystem services, global mangrove cover has remarkably declined in the recent decades. While many mangrove rehabilitation efforts have been implemented worldwide, many attempts show negative results. These efforts often fail because the lack of understanding the local natural system.

One example of a mangrove-mud coastal system undergoing such mangrove degradation is the highly dynamic system of the Guyana Shield in the northern coast of South America. This dynamic system is known for its offshore migrating mudbanks that play a crucial role in facilitating natural cycles of shoreline expansion and mangrove colonisation. Due to among others the construction of grey-engineering coastal protection and land-use for aquaculture, mangrove degradation accelerates in this area. In order to apply appropriate mangrove rehabilitation methods, the natural system of the Guyana Shield needs to be understood well. Existing studies mainly focussed on site specific problems, leaving a knowledge gap on how to efficiently coordinate mangrove restoration efforts within the larger scale influenced mud dynamics.

This research, therefore, seeks to connect that knowledge gap by exploring how the mudbank morphology and related hydrodynamics influence the effectiveness of sediment-based mangrove rehabilitation methods in the Guyana Shield. The main objective is to identify which type of restoration measure are the most appropriate at the different stages of the mudbank-migration-cycle and to develop a framework that can guide the timing and spatial planning of these sediment-based interventions. To reach this goal, the study combines a extensive literature study, a case study, a conceptual model, and a process-based numerical model.

The study starts with an extensive review of the current state of knowledge on mud dynamics, mangrove ecology, and mangrove rehabilitation techniques. The review highlights the importance of understanding the influence of hydrodynamic forcing as waves and tidal dynamics on sediment transport, the species-specific thresholds for establishment for mangroves, and the importance of applying system-specific approaches for successful mangrove restoration. The insights gained from this review were then applied on the case study, the Guyana Shield. The area experiences a semi-diurnal tidal regime, and is mostly influence by the north-eastern trade winds. Together with the high sediment transport rates originating from the Amazon River there is a net-transport direction westward. This sediment transport is characterised by the movement of offshore migrating mudbanks, creating alternating phases of coastal accretion and erosion along the coast of the Guyana Shield.

Using information from both the literature study and existing pilot projects in the region, a conceptual model is constructed to identify the most relevant variables in the system. These include the hydrodynamic variables, morphological variables, biological and social variables, and lastly the intervention design variables. This conceptual framework serves as a basis for the input for the process-based numerical model using Delft3D, which is used to simulate the different stages of the phases representing the mudbank-migration cycle. From this conceptual framework it becomes clear that the entire system is influenced by the phases related to the offshore mudbank movement and the associated wave conditions. It is assumed that the peak sediment supply and the maximum wave conditions are out of phase, and that the response of the bed level, and therefore also of the mangroves, lags behind the influence of the mudbanks. The presence of a mudbank results in high sediment input and offshore wave dampening, creating favourable conditions for sediment deposition and shoreline expansion, allowing mangroves to colonise.

The Delft3D modelling research was used to test two sediment-based interventions meant for mangrove rehabilitation: by constructing an artificial chenier, and by implementing a mud motor. Both interventions were placed at a different stage in the mudbank-cycle, both intended to stabilise the nearshore zones, either by keeping the sediment there or by promoting sediment transport to the coast. The modelling results show that the effectiveness of these restoration strategies are closely tied to the phase of the mudbank cycle. During the transition from a mudbank to an interbank phase, cheniers can create sheltered conditions that promote sediment accumulation in the nearshore and allow the cheniers to migrate landwards, assuming it proves mangroves a stable substrate for mangrove establishment. For chenier placement, results prove that placing the chenier in the intermediate zone (in this research placed at 2600 m from shore) provides the best outcome in stabilising the nearshore zone. In contrast, in the transition phase from interbank to mudbank, the mud motor proves to be effective by delivering the sediment to the shore and accelerating the bed level growth. For the mud motor results show that the closer the mud is placed to the coast, the more effective the outcome.

The findings in this research confirm that mangrove restoration cannot be approached with a one-fits-all strategy. Instead, the method, timing, and location of the intervention must be carefully aligned with the natural cycle of the mudbank dynamics of the coastal system. By combining the conceptual system understanding with numerical simulation, this research provides a basis for a practical approach for nature-based solution as coastal protection within the Guyana Shield.